1. Field of the Invention
The present invention is in the field of preparing fluorescent material of small particle size suitable for use on fluorescent surfaces of high luminance and high fineness cathode ray tubes such as those used, for example, in airplane cockpits or in television projectors.
2. Description of the Prior Art
Color cathode ray tubes used, for example, in cockpits of airplanes or for color television projectors must have a high resolution power, a high degree of fineness, and high luminance. Generally, the fluorescent surface of a color cathode ray tube is coated in a predetermined pattern such as a stripe or dot pattern with fluorescent materials corresponding to the primary colors, in a narrow width. As the resolution power and the fineness have been increased, a higher density is required for the pattern. A particle size of about 10 microns (um) which is typical for the usual fluorescent material particles is too large to obtain an optimum coating thickness for the fluorescent material necessary for attaining high luminance. In view of the foregoing, it is necessary to prepare fluorescent material of small particle size in order to obtain a color cathode ray tube of high luminance and a high degree of fineness.
Fluorescent materials of small particle size have been made by several processes in the prior art. A first method involves reducing the amount of flux added to the fluorescent material or avoiding the use of the flux altogether upon forming the fluorescent material, thereby suppressing crystal growth.
A second method involves filtering or sieving fluorescent material of small particle size separated from fluorescent material produced by the usual method.
A third method consists of pulverizing fluorescent material produced in the usual manner into a smaller particle size.
The first method involves the disadvantage that the particle growth remains in an intermediate and thus unstable stage. A desired fluorescent product cannot be obtained as a single phase which reduces the luminance.
The second method is difficult to carry out and provides an extremely low yield.
The third method involving pulverizing is not very acceptable because pulverizing injures the particle surface so as to reduce the luminance as much as from 30 to 40% of the inherent luminance.
When the fluorescent pattern for each of the colors is made finer, the luminance tends to be lowered in the formation of the fluorescent surface due to the packing density of the fluorescent particles in each of the portions and accordingly, a higher luminance is required of the fluorescent particles.